Add debug selinux permission to write saved_cmdlines_size.

Now that we're expected to use this when taking traces, we need to add
this permission so that Traceur can also access this file.

Test: Used Traceur and saw the traces appear in the bugreports
directory, as expected.
Bug: 62493544

Change-Id: Ib4304176abbb51e2e3b45c566ff14574e1cfaa82
Merged-In: I464b0df30fabfc5f1c7cd7430e53e8d04bfacb53
(this merged-in is not the same change; it's a conflicting change in
master)